Electronic components, such as semiconductor substrates or packaged semiconductor devices in the form of Quad Flat No-Lead (QFN) packages and Chip-Scale Ball Grid Array (CSBGA) packages, usually undergo singulation into separate units after they have been packaged in an array arrangement. After sawing, the molded surfaces of the singulated electronic units will be contaminated with saw residue, such as saw dust and copper traces.
Typically, singulated electronic packages may be cleaned by mechanical agitation. In one prior art U.S. Pat. No. 6,446,354 entitled “Handler System for Cutting a Semiconductor Package Device”, a soft brush is used for creating mechanical agitation on the bottom molded surfaces of the packages. This action assists in removing the saw residue by loosening the residue. The brush may be wet when wet brushing is required. Next, high pressure water jets are directed at the bottom molded surfaces of the packages to wash off and remove the loosened saw residue.
There are disadvantages in using a brush for cleaning by mechanical agitation. For example, after a long period of use, some saw residue is trapped in the brush. Periodic maintenance is therefore necessary to keep the brush clean, or otherwise the dirty brush may introduce dirt onto the surfaces to be cleaned instead. The cleaning effect by mechanical agitation will also be largely reduced with a dirty brush. Additionally, since there is actual contact with the molded surfaces of the packages during brushing, care must be taken not to damage the packages. While the brushing force acting on the molded surfaces must be sufficiently large to loosen the saw residue, the force should not be too large to dislodge the packages being held by vacuum on a pickhead during washing. Further, the pressure from the water jets must be sufficiently high for washing off the loosened saw residue but this must not be so large as to dislodge the singulated electronic packages.
To avoid the disadvantages of mechanical agitation, U.S. Pat. No. 5,339,842 entitled “Method and Apparatus for Cleaning Objects” discloses the use of megasonic vibrations to enhance cleaning of electronic packages. Megasonic cleaning uses acoustic frequencies of approximately 800 KHz to 1.8 MHz. Therefore, megasonic cleaning can be highly effective for removing particles having a particle size of about 1 micron or less. In this cleaning method, the bottom surface of a workpiece is cleaned by immersing the workpiece in a first water tank overflowing with water such that the bottom surface of the workpiece is in contact with the surface of the running water while the workpiece is moved through the tank. At the bottom of the water tank, a transducer generates megasonic waves that propagate upwardly through the water to the surface of the water where the workpiece is moving through. The flowing water and the megasonic waves loosen the saw residue on the bottom surface of the workpiece, and the water carrying the loosened saw residue flows into a second water tank surrounding the first water tank to be collected. However, since megasonic vibrations are high frequency waves which are highly focused in nature, only a limited area of the workpiece can be cleaned. To subject the entire workpiece to the cleaning effect of the megasonic vibrations, the routing distance of the workpiece is long and time consuming to implement. Furthermore, the transducer is enclosed by a housing which causes some megasonic energy loss and thus reduces the cleaning effect.
US Publication No. 2009/0038638 A1 entitled “Megasonic Cleaning System” discloses an improvement over the above prior art in that the workpiece is cleaned by a linear arrangement of nozzles, with each nozzle housing a megasonic transducer. Megasonic vibrations are transmitted to each water jet passing through the nozzles. The actuated water jets clean the molded surface of the workpiece with the vibrational energy of the megasonic wave. Relative movement between a pickhead supporting a workpiece and the nozzle assembly with multiple transducers in X and Y axes respectively allow the entire surface of the workpiece to be cleaned expeditiously.
However, his approach has the same disadvantage as with U.S. Pat. No. 5,339,842 in that each transducer is housed within a nozzle body and is enclosed. Some megasonic acoustic energy is lost to the inner surface of the nozzle when the water jet passes through it or when there is a change in directional flow within the nozzle. This reduces the effectiveness of cleaning using the actuated water jets. Furthermore, the size of each nozzle limits the number of nozzles that can be laid out and therefore the number of transducers that can be accommodated in the arrangement of water jets. As a result, the transducers are located relatively far apart, and hence the nozzle assembly needs to travel a longer distance to cover the entire length of the workpiece.
As the nozzles are arranged in a single line, more time is required to move the pickhead through the entire width of the workpiece so that all areas of the workpiece may be cleaned. Thus, while cleaning time is reduced over U.S. Pat. No. 5,339,842, substantial time is still required for moving the pickhead and the nozzle assembly to cover all areas of the workpiece. A faster cleaning process is preferred, particularly when handling larger electronic devices such as BGA devices which require much less singulation time since faster cutting speeds can be achieved and fewer units are being handled. A time-consuming cleaning process will thus cause a more pronounced delay in the in-line operation when handling singulated BGA devices.
Therefore, it would be desirable to achieve a cleaning method for singulated electronic packages which sufficiently cleans the packages without damaging or loosening any singulated units and which can be completed within a shorter time.